How to create many clips in the NLA editor
The official documentation for the NLA Editor gives you the basics of operating the UI.

How to incorporate this technology into your workflow is a little more complicated.  Likewise choosing which keyframes belong in which NLA strips depends on your experience and understanding of the mathematics of the NLA system.  You're just going to have to experiment and fool around and make plenty of mistakes.  

Do not underestimate the value of setting aside time just to play around and make mistakes and then try to fix those mistakes under no time pressure and with no milestones or assets at stake.

My explanation of how to split an action into two separate NLA strips probably will be somewhat confusing on first read, but after some experimentation you should have the necessary experience to understand it and absorb the concepts within.